Mapping of electrostatic and magnetic forces
If the tip of atomic force microscopy is located at rather large distance from the surface
(i.e. 20 nm), then the intesity of interatomic forces is small. Long-range forces which there predominate are of electrostatic origin
or magnetic if we work with feromagnetic materials.
The microscope can easily be used for volume mapping such forces. In such cases we refer to EFM - electric force microscopy or
MFM - magnetic force microscopy.
For measurement of electrostatic forces it is sufficient to use conductive tip similarly to the measurement of
local electric current (even the electric circuit is the same, but the tip is not in contact with the surface).
The tips are mostly made of dopped silicon or covered with thin conductive layer, for example platinum.
Special type of measurement is mapping of contact potential by use of alternating voltage between the tip and surface while subtracting
the DC component of voltage on the tip (KPFM - Kelvin probe force microscopy) and capacity measurement of the tip - surface in
capacity microscopy (SCM - Scanning capacitance microscopy).
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Measurement of field intensity is measured in several levels above the sample which enables to
acquire more data for eventual reconstruction of charge distribution or sample magnetization and also it is possible
to estimate influence of other forces between tip and the sample.
Pro měření magnetických sil je nutné použít hrot pokrytý magneticky tvrdým materiálem a před měřením jej
zmagnetizovat, což se děje cívkou, nebo permanentním magnetem. V některých mikroskopech je možné aplikovat magnetické
pole na vzorek, většinou pomocí dlouhých pólových nástavců elektromagnetu umístěného stranou od vzorku.
Mikroskopie magnetických sil nachází uplatnění především při studiu záznamových médií.
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| Schema AFM při měření proudu a mapování elektrických polí |
Quantitative measurements
Method with the easiest interpretation of data is probably the measurement of mapping of contact potential, where
the measured quantity is directly the voltage by which it is necessary to compensate the contact potential. If the surface does not
contain complicated morphological structures, the results can be used directly. Other methods are more difficult to use, among other
uncertainity sources of electrostatic measurements these should be taken into account:
- the influence of long range van der Waals forces
- the influence of the probe itself to the distribution of electrostatic field
Also in this situation it is the best method of data interpretation to first calculate
distribution of electrostatic field between the probe and the surface for each pixel of resulting image.
Nevertheless, this is numerically somewhat uneasy.
At measurement of magnetic fields the situation is even more complicated, except the uncertainitied mentioned above
(in magnetic microscopy we should also take into account acting of the electrostatic forces) these effect play the role:
- rather unknown magnetic properties of the tip and their changes during the measurement
- ambiguous dependence between magnetization and field distribution (it is complicated to calculate magnetization from the given field).
Quantitative magnetic microscopy is therefore still very difficult metrology area.
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